A network structure of Next Generation Network (NGN) is shown in FIG. 1, in which Media Gateway Controller (MGC) 110 and Media Gateways (MGs) 121, 122 are two key components in NGN. MGC provides the function of call control and MG provides the function of service bearing, thus the separation of Call Control Plan and Service Bearing Plan can be realized, network resources can be shared to full advantage, upgrading of devices and extending of service can be simplified, and the cost of development and maintenance can be reduced greatly.
Media gateway control protocols are primary communication protocols between MG and MGC, and at present, two media gateway control protocols widely used are H.248/MeGaCo (Gateway Control Protocol) and Media Gateway Control Protocol (MGCP). MGCP is drafted in October, 1999 and revised in January, 2003 by Internet Engineering Task Force (IETF), and H.248/MeGaCo is drafted in November, 2000 and revised in June, 2003 by IETF and International Telecommunication Union (ITU) jointly.
Take H.248 for example, all kinds of resources on MG are abstractly expressed as Terminations. Terminations can be divided into physical Terminations and ephemeral Terminations; the former represents physical entities with a half-permanent existence, such as a Time Division Multiplex (TDM) channel, while the latter represents common resources applied for use ephemerally and released after the duration of their use, such as Real-time Transport Protocol (RTP) streams. The association between Terminations is abstractly expressed as Context. The Context can involve a plurality of Terminations; therefore the Context describes the relationship between the Terminations in Topology.
Based on the abstract model of the protocol, connection of calls is actually manipulating of Terminations and Contexts. The manipulating is implemented through request and response of Commands between MGC and MG. The parameters carried by a Command are also called Descriptors, which are divided into several classes such as Property, Signal, Event, and Statistic Descriptors etc. The parameters with service correlation are logically converged into a Package.
The topological structure of Context defined by H.248 is not sensitive to the type of Termination itself, i.e. the Terminations composing a Context can be any physical Terminations (such as TDM channels) or ephemeral Terminations (such as RTP streams). With respect to a bidirectional media stream, common IP telephone service can be carried by interconnecting a TDM physical Termination and a RTP ephemeral Termination, local TDM telephone service can be carried by interconnecting two TDM physical Terminations, and IP-IP interconnection service can be carried by interconnecting two RTP ephemeral Terminations.
The original model of H.248 protocol is designed for MGC-MG control association inside a single IP domain. In general, a media stream is transferred between a TDM physical Termination and a RTP ephemeral Termination on common MGs. But in practical applications, besides the interconnecting of MGs under the control of MGC within respective IP domains, operators possibly need to realize interconnection between different IP domains with each other.
However, RTP streams over different IP domains need to be connected in series in the same Context to realize interconnection between IP domains; for this purpose, an MG creating a RTP stream needs to learn the information of the corresponding IP domain, so as to determine the direction of the media stream. This is to say, if the MG can not obtain the corresponding IP domain information, the RTP stream can not be created at all. At present, not any technological method can make an MG which is required to implement interconnection obtain IP domain information for creating RTP streams, i.e. at present, IP domain information for creating RTP streams can not be obtained on MG.